[PATCH 1/3] riscv: optimized memcpy
David.Laight at ACULAB.COM
Tue Jun 15 06:18:33 PDT 2021
From: Bin Meng
> Sent: 15 June 2021 14:09
> On Tue, Jun 15, 2021 at 4:57 PM David Laight <David.Laight at aculab.com> wrote:
> > I'm surprised that the C loop:
> > > + for (; count >= bytes_long; count -= bytes_long)
> > > + *d.ulong++ = *s.ulong++;
> > ends up being faster than the ASM 'read lots' - 'write lots' loop.
> I believe that's because the assembly version has some unaligned
> access cases, which end up being trap-n-emulated in the OpenSBI
> firmware, and that is a big overhead.
Ah, that would make sense since the asm user copy code
was broken for misaligned copies.
I suspect memcpy() was broken the same way.
I'm surprised IP_NET_ALIGN isn't set to 2 to try to
avoid all these misaligned copies in the network stack.
Although avoiding 8n+4 aligned data is rather harder.
Misaligned copies are just best avoided - really even on x86.
The 'real fun' is when the access crosses TLB boundaries.
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